Ive found somewhere between 25-35 is the sweet spot
To determine the total cost of a spell, start with the mana cost or alternative cost you're paying, add any cost increases, then apply any cost reductions (such as that of Serpent of the Pass's second ability)
If you're hoping to get your hands on one of these cards, they can only be pulled from Collector Booster packs, and if recent availability is anything to go by, this product will sell out long before the set is even released, so be sure to get pre-orders in as soon as possible
One of the most notable Dragon-type VSTAR Pokmon is Hisuian Goodra VSTAR
There are also Special Energy cards, like the popular Double Colorless Energy, but each Special Energy is restricted to 4 per deck, as opposed to an open-ended number of Basic Energy